1. Field of the Invention
The present invention relates to a semiconductive device and a method for fabricating the same. More particularly, the present invention relates to a metal-oxide-semiconductor (MOS) transistor and a method for fabricating the same.
2. Description of the Related Art
In a typical MOS process, a gate oxide layer and a poly-Si layer are sequentially formed on a substrate, and then the poly-Si layer is patterned into a gate using lithography and etching techniques. Thereafter, ion implantation is conducted to form a source/drain region in the substrate beside the gate.
The conventional material of MOS gates is doped poly-Si. However, poly-Si is not an ideal gate material in advanced processes for having higher resistance and easily inducing gate depletion to cause voltage loss. Therefore, metal is currently used to form the gates in many advanced MOS processes. However, since the metal atoms in a metal gate easily diffuse to, for example, the channel region, the performance of the MOS transistor is not reliable.
On the other hand, there exist optimal wok functions for an NMOS gate and a PMOS gate, respectively. When the work functions of the NMOS gate and the PMOS gate are not optimized respectively, the performance of the CMOS device cannot be well adjusted. Therefore, it is highly desired to develop a method for controlling the work functions of different types of MOS gates.